Thermal transfer printers which are adapted for use with plain paper, having specified surface smoothness and serving as a recording medium, for transferring a sublimable dye or ink to the paper by a thermal head with application of pressure and heat are known. Such thermal transfer color printers are used also for producing color images.
These thermal transfer color printers include those adapted to produce color images by repeatedly transferring usually yellow, magenta and cyan inks in succession for each image to be eventually produced. (these types of printers will hereinafter be referred to as the "successive color transfer type.")
With printers of the successive color transfer type, it is essential to assure color registration with improved accuracy and an increased throughput. For this purpose printing systems and paper or ink transport systems of different types have been proposed.
For example, a pin feed type transport system is known wherein pin sprockets are used for transporting fanfold paper serving as a recording medium. Cut sheets of paper or roll paper is transported by friction rollers, such as platen roller, backup roller and like transport rollers, utilizing the friction afforded by the nip of the rollers.
With the printer of the successive color transfer type, the thermal head is pressed against a platen roller in the form of a hollow cylinder under a predetermined pressure with the recording medium, i.e. printing paper, interposed therebetween, and ink is thereafter transferred from an ink film onto the printing paper by heating the thermal head. This process is repeated for each color, i.e., yellow, magenta and cyan. To superimpose the colors by transfer, the printing paper having one color transferred thereto is reversely transported to the transfer start position on the platen roller for the transfer of another color.
The cylindrical platen roller is made of an elastically deformable member so as to assure proper contact between the thermal head, the ink film and the printing paper.
However, since the platen roller is elastically deformable to produce great friction drag between the paper and the platen roller, a difference in paper transport force is likely to occur between the platen roller and the paper feeder, subsequently causing the position one color to be out of register with another color upon transfer. To avoid such a failure in color registration, the paper is transported at a low speed.